An image forming apparatus using an electrophotographic process, e.g., a laser beam printer, comprising a fusing device which thermal-fuses a toner image formed on a printing medium (e.g., a printing sheet or OHP sheet). A heating system which can be used for the fusing device includes several types. Of these types, an electromagnetic induction heating system which induces a current in a fusing roller using a magnetic flux and generates heat using the resultant Joule heat, in particular, can directly cause the fusing roller to generate heat by using the generation of the induced current. This system is advantageous over a fusing device based on a heated roller system using a halogen lamp as a heat source in terms of achieving a high-efficiency fusing process (see, for example, Japanese Utility Model Laid-Open No. 51-109739).
Recently, a color image forming apparatus (A4 apparatus) capable of printing on standard-sized sheets, e.g., A4 size sheets, at a rate of 16 sheets/min has been able to implement a technique of heating the roller only at the time of printing. This is often referred to as “on-demand fusing”, which uses a fusing device with a small heat capacity based on the above electromagnetic induction heating system so that no fusing temperature control is required during standby.
On the other hand, in a color image forming apparatus (A3 apparatus) capable of printing on standard-sized sheets up to A3 size, the fusing device is generally required to have a larger heat capacity than the fusing device in an A4 apparatus, although it depends on the printing speed. This apparatus therefore performs preheating by supplying power to the fusing device at predetermined time intervals even during standby, i.e., so-called “standby temperature control” (see, for example, Japanese Patent Laid-Open No. 2002-056960). The following is the reason why standby temperature control is performed.
FIG. 27 shows, for a color image forming apparatus (A3 apparatus) using a fusing device based on a conventional electromagnetic induction heating system, the relationship between the start-up time required for the temperature of the fusing device in a cooled state to reach a temperature at which printing can be done (e.g., 180° C.) and the corresponding power (fusing power) supplied to the heater of the fusing device. Referring to FIG. 27, if the fusing power that can be supplied is about 900 W, the start-up time required to reach a temperature at which printing can be done (print temperature) is 30 sec (point Wa). This time is much shorter than the start-up time required in a commonly used fusing device using a halogen heater. However, if we consider the sheet convey time and the like, the time (first printout time) between the instant at which printing is started and the instant at which the first image-bearing sheet is discharged to a paper discharge unit increases to more than 30 sec, thus making the user wait. For this reason, in order to shorten the first printout time, power is supplied to the fusing device at predetermined time intervals even during standby to perform preheating (as generally done in an image forming apparatus using a fusing device based on the halogen heater system). Executing this standby temperature control makes it possible to quickly reach a predetermined fusing temperature, at which image forming can be performed, once a printing job is started.
The power consumption at the time of standby temperature control in the electromagnetic induction heating system can be suppressed low because the temperature at the time of standby temperature control can be set to be lower than that in the fusing system using a halogen heater. As compared with the on-demand fusing system, however, this system still requires extra power (power at the time of standby temperature control).
In this image forming apparatus, if the power supplied to the heater of the fusing device can be increased by about 200 W, a power of 1,100 W can be supplied to the fusing device, and the time taken to reach the print temperature becomes about 15 sec (a point Wb in FIG. 27). If, therefore, the target first printout time for this image forming apparatus is about 20 sec, on-demand fusing which requires no standby temperature control can be realized (although it depends on the arrangement, the paper convey paths, the convey speed, and the like of the image forming apparatus).
With the recent technical improvements in image forming apparatuses, even image forming apparatuses in the category of medium-speed apparatuses (middle-class apparatuses) have been reduced in size and cost and increased in speed. The printing speeds of such apparatuses have reached those of high-speed apparatuses a decade ago. Along with this tendency, the market has further demanded value added such as energy saving and a reduction in first printout time.
In light of this, even by using a fusing device based on the high-efficiency electromagnetic induction heating system or on-demand fusing, which has been implemented in conventional A4 apparatus, has become difficult to meet such market demands.
As described above, in an A3 apparatus using conventional standby temperature control practice, power is supplied to the fusing device during standby even though the necessary power is minimum. Therefore, this standby temperature control constitutes one of the factors that makes it difficult to reduce the power consumption of the image forming apparatus during standby.
However, in the case where power saving is important during standby and the standby temperature control is not executed, it takes more time to reach a predetermined fusing temperature, at which image forming can be done. As a consequence, another problem arises, that is, the first printout time becomes longer. In other words, there is a tradeoff between energy saving during standby and a reduction in first printout time.
An on-demand fusing system balancing energy saving during standby and reducing the first printout time, which comprises a short temperature rise time suited for the market levels needs to be developed.
Although a large-size, high value-added image forming apparatus such as high-speed monochrome printing apparatuses or high-quality color printing apparatuses, i.e., so-called high-speed apparatuses (high-class apparatuses), are devised to save energy, but also comprise value added such as high performance devices and abundant optional supply of equipment. That is, there is a tendency toward increasing power consumption. One of the criteria for determining the upper limit of the power consumption of such an apparatus is the maximum current that can be supplied by commercial power supplies. Assume that a maximum supply current of 15 A is specified for a 100-V commercial power supply. In this case, the upper power limit is 1,500 W (=100 V×15 A). An image forming apparatus is generally designed such that the maximum current, that the apparatus requires, does not exceed the maximum current of the commercial power supply.
For high-speed apparatus class fusing devices, a fusing device with a larger heat capacity is generally used to stand high-speed continuous fusing. The inconvenience of such a fusing device is that it takes a long period of time (several minutes) (warm-up time) for the temperature of the fusing device, in a cooled state, to reach a temperature in a standby state. One of the challenges to overcome this is to shorten the warm-up time.
Assume that the warm-up time of the fusing device is to be shortened by simply supplying large power. In this case, since the maximum power of the commercial power supply defines the upper power limit that can be used, it is difficult to further shorten the warm-up time unless the fusing device itself is improved.
For example, as a proposal to solve such a problem, Japanese Utility Model Publication No. 7-41023 discloses that in order to effectively use power for a fusing device, an image forming apparatus whose fusing device includes a main heater and a sub-heater is provided with a rechargeable battery unit, and the rechargeable battery unit is designed to selectively connect to a DC power supply or DC motor control unit. More specifically, while the rechargeable battery unit is supplying power to the DC motor, power that should be supplied to the DC motor can be supplied to the sub-heater, and hence the temperature of the fusing device can be raised higher than in the prior art. During this period, copying can be done at high speed.
In addition, Japanese Patent Laid-Open No. 2002-174988 discloses a method of achieving energy saving and a reduction in print start time by providing a rechargeable battery device for an image forming apparatus and using both power from a commercial power supply and power from the rechargeable battery device during startup of a fusing device.
According to the arrangements disclosed in Japanese Utility Model Publication No. 7-41023 or Japanese Patent Laid-Open No. 2002-174988, since the power supplied from the rechargeable battery means to the sub-heater or a predetermined load is simply turned on/off, the maximum power that can be supplied from the commercial power supply may not be effectively used depending on the voltage of the commercial power supply to which the image forming apparatus is connected to or the load condition of the image forming apparatus. In addition, the arrangement of the fusing device is complicated because it requires a plurality of heaters.
Furthermore, in an image forming apparatus whose fusing device includes a main heater and a sub-heater, when the fusing device is to be started up without sufficient power stored in the rechargeable battery device, there is a chance that no power will be supplied to the sub-heater or the loads of the image forming apparatus other than the fusing device. If no power can be supplied to the sub-heater, the sub-heater portion will also be heated by the main heater. Thus, it may require longer startup time than in a conventional fusing device having no rechargeable battery device. Furthermore, if the required power cannot be supplied to the loads of the image forming apparatus other than the fusing device, the image forming apparatus may not normally operate.